Anal. Chem. 2005, 77, 2218-2226
Single-Photon Ionization Quadrupole Mass Spectrometry with an Electron Beam Pumped Excimer Light Source F. Mu 1 hlberger,*,† J. Wieser,‡ A. Morozov,§ A. Ulrich,§ and R. Zimmermann†,|,⊥
Institut fu¨r O ¨ kologische Chemie, GSF-Forschungszentrum fu¨r Umwelt und Gesundheit, D-85764 Neuherberg, Germany, and TuiLaser AG, D-82110 Germering, Germany, Fakulta¨t fu¨r Physik E12, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany, Institut fu¨r Physik, Universita¨t Augsburg, D-86159 Augsburg, Germany, and BIfA GmbH, D-86167 Augsburg, Germany
The application of soft ionization methods for mass spectrometry (MS), such as single-photon ionization (SPI) using vacuum ultraviolet (VUV) light, provides powerful analytical instrumentation for real-time on-line monitoring of organic substances in gaseous matrixes. A compact and mobile quadrupole mass spectrometer (QMS) system using a novel electron beam pumped rare gas VUV lamp for SPI has been developed for on-line analysis of organic trace compounds (ppb concentrations). The VUV radiation of the light source is employed for SPI in the ion source of the QMS. The concept of the interfacing of the VUV light source with the QMS is described and the SPI-QMS is characterized. On-line detection limits down to 50 ppb for benzene, toluene, and m-xylene were achieved. The instrument is well suited for continuous measurements of aromatic and aliphatic trace compounds and can therefore be used for on-line monitoring of trace compounds in dynamically fluctuating process gases. First measurements of gas standards, petrochemical samples, and on-line monitoring of automotive exhaust are presented. Up to now trace components of process gases have been measured by means of off-line analytical methods in many industries. Therefore, samples have to be taken, enriched, prepared by wet chemistry methods, and applied to an instrumental analysis, e.g., gas chromatography/mass spectrometry (GC/MS).1 In this case, the different compounds are commonly separated according to their volatility by gas chromatography. With GC/MS systems, the separated substances are detected by electron impact-mass spectrometry (EI-MS). The individual mass spectra show specific fragmentation patterns that can be assigned to specific substances. The procedure, however, is time-consuming, and therefore, the method is not well suited for on-line process analysis. An analytical on-line method preferably should avoid * Corresponding author. E-mail:
[email protected]. † GSF-Forschungszentrum fu ¨ r Umwelt und Gesundheit. ‡ TuiLaser AG. § Technische Universita¨t Mu ¨ nchen. | Universita¨t Augsburg. ⊥ BIfA GmbH. (1) Holmes, J. C.; Morrell, F. A. Appl. Spectrosc. 1956, 11, 86-87.
2218 Analytical Chemistry, Vol. 77, No. 7, April 1, 2005
preconcentration steps. For on-line measurements of complex gases containing organic compounds, ionization methods with a low degree of fragmentation (soft ionization) are required in order to avoid spectra with overlapping peaks. Fragment peaks would make the interpretation of spectra difficult if not impossible. Single-photon ionization (SPI) with vacuum ultraviolet (VUV) light allows an efficient ionization of organic compounds.2 Selectivity is provided via the ionization potential (IP), as only compounds with an IP lower than the photon energy can be ionized. Due to the small amount of excess energy transferred (i.e., energy exceeding the ionization potential) SPI is a soft ionization method. With the SPI technique, mass spectra show no or few fragment peaks. VUV light for SPI can be generated by intense short pulse lasers. A wavelength of 118 nm (10.49 eV), generated by frequency tripling of intensive 355-nm third harmonic Nd:YAG UV laser pulses in a rare gas cell,3-5 is often used for SPI. The time width of such VUV light pulses (and therefore the duration of the ionization process) is in the range of some nanoseconds, resulting in similar short ion pulses well suited for time-of-flight mass spectrometry. Laser-based SPI can be employed in research6 and development. Industrial application, however, suffers from the fact that laser-based SPI requires an expensive and sophisticated pulsed laser system. In principle, single-photon ionization mass spectrometry can also be performed using VUV lamps, e.g., deuterium lamps, as the photon source. However, the achievable VUV photon density outside such lamps is relatively weak in the required spectral range. Thus, conventional VUV lamps are not well qualified for sensitive on-line analysis of trace compounds. Since 1958 the VUV emission of rare gas excimers has been known.7 Previously it could be shown that low-energy (10-15 keV) electron beam excitation of dense gases leads to strong light (2) Tonokura, K.; Nakamura, T.; Koshi, M. Anal. Sci. 2003, 19, 1109-1113. (3) Bjorklund, G. C. IEEE J. Quantum Electron 1975, QE-11, No. 6, 287296. (4) Maker, P. D.; Terhune, R. W. Phys. Rev. 1965, 137 (3A), 801-818. (5) Vidal, C. R. In Tunable Lasers; Mollenauer, L. F., White, J. C., Eds.; SpringerVerlag: Berlin, 1987; Vol. 59, pp 56-113. (6) Mu ¨ hlberger, F.; Hafner, K.; Kaesdorf, S.; Ferge, T.; Zimmermann, R. Anal. Chem. 2004, 76, 6753-6764. (7) Tanaka, Y.; Jursa, A. S.; Blanc, F. J. J. Opt. Soc. Am. B 1958, 48, 304. 10.1021/ac048319f CCC: $30.25
© 2005 American Chemical Society Published on Web 02/23/2005
Table 1. Center Wavelength, Photon Energy at Center Wavelength, and Emission Bandwidth of Different Rare Gases and Gas Mixtures
a
source of light
gas medium
Ar2* Kr2* Xe2* Ar* and NeAr*
Ar Kr Xe Ne + Ara
Kr* and ArKr*
Ar + Kra
Xe* and KrXe*
Kr + Xea
H* O*
Ne + H2a Ar + O2a
N*
Ar + N2a
center wavelength (nm)
photon energy at center wavelength (eV)
126 150 172 104.8 106.7 116.5 123.6 129.6 147.0 121.6 130.2 130.5 130.6 149.26 149.28 149.47 174.3 174.5
9.8 8.3 7.2 11.83 11.62 10.64 10.03 9.57 8.44 10.18 9.52 9.50 9.49 8.308 8.307 8.296 7.11 7.11
bandwidth (nm) 9 11 14 depends on Ar pressure depends on Kr pressure13 depends on Xe pressure13
